There are numerous situations wherein a fluid medium is contaminated by particulate substances, especially ferrous particles. Frequently it is desired to determine the presence of such substances. Data are generally acquired which indicates the presence of particles. The data are used either as an indication of the condition of the fluid which in turn may serve as an indication of a possible maintenance problem with the apparatus being served by the fluid. Of course, the data may be used to monitor the condition of the fluid itself. In highly critical applications such as in the lubricating of helicopter and aircraft engines and transmissions, where the opportunities for maintenance may be quite limited, it is further desirable to be able to discriminate between minor particles ("fuzz") resulting from normal wear and larger individual particles indicative of abnormal conditions, such as cracks or impending fatigue failure. Lastly, it is very desirable that the observer or pilot be able to distinguish between random occurrences of individual particles and a steady buildup of such particles, that is, to measure the rate of particle accumulation.
A great number of devices have been developed to filter or trap loose particles in engine coolant and/or lubrication systems. In many of the devices designed for ferrous particles, such as those described by Lammers in U.S. Pat. No. 3,421,627 and Tauber in U.S. Pat. No. 3,753,442, this is done by attracting the particles to the poles of a magnet. In more advanced versions of these devices, such as those described by Botstiber in U.S. Pat. Nos. 2,983,890 and 3,317,042, the magnet is combined with a signaling device designed to indicate the presence of either a significantly large particle or a large collection of smaller particles. This acts to warn the pilot of a possible impending failure in time for corrective repairs to be applied. However, these devices provide no information as to whether the source of the signal is an innocuous accumulation of normal wear fuzz, a large random sliver formed by or introduced during some previous maintenance operation or an accumulation of 100+ micron particles which have been found to be highly significant indicators of crack formation and growth. Furthermore, such devices provide no information as to the rate of particle accumulation so it is not really possible for the user either to assess the significance of the warning signal or to estimate with any reliability the remaining useful lifetime of the system.